Ultrasonic excitation by amplitude-modulated diode laser

A laser-ultrasonic transducer was newly developed for the purpose of exciting ultrasonic wave in UHF range with flat frequency response. An infrared semiconductor laser whose output power is modulated with sinusoidal driving current illuminates a thin metal film formed on a flat surface of a solid rod immersed in a sample liquid. The heat generated in the film causes periodical thermal expansion, which effectively emits ultrasonic waves into the liquid. The absence of electrical or mechanical resonance in the process of ultrasonic generation provides us with an ideal flatness in the frequency response of ultrasonic excitation over the range of 30-120 MHz. Ultrasonic propagation in the liquid was observed by the optical beating Brillouin scattering technique at frequencies over 100 MHz. The intensity of the excited ultrasonic wave is about 10/sup 4/ times larger than that of the spontaneous phonon and the sensitivity of the Brillouin scattering measurement was remarkably increased.